It is quite common for a DBA or consultant to setup database mirroring or an always on availability group and then leave the organization.
When it is time to clean up SQL Server logins on your SQL Server you discover, that although you have revoked access for the login to all databases and server roles, you are still unable to delete the login as it is reported to own an endpoint.
Below shows the error reported in SQL Server 2014.
To confirm the endpoint owner you could use the following:
USE master; SELECT SUSER_NAME(principal_id) AS endpoint_owner ,name AS endpoint_name FROM sys.database_mirroring_endpoints;
USE master; ALTER AUTHORIZATION ON ENDPOINT::Mirroring TO sa;